Add silenced? for Action View Start subscriber

Previously, the `render_template.action_view` and
`render_layout.action_view` events would always be handled as if they
had subscribers even if the log level of its subscribers result in
nothing being logged. For regular `LogSubscriber`s,
`subscribe_log_level` could be used to optimize these cases but the
Start subscriber is not a subclass of `LogSubscriber`.

This commit implements a `#subscribed?` method for the Start subscriber
so that it can also benefit from the `subscribe_log_level` optimization.

actionview/lib/action_view/log_subscriber.rb

@@ -96,6 +96,10 @@ module ActionView
 
       def finish(name, id, payload)
       end
+
+      def silenced?(_)
+        logger.nil? || !logger.debug?
+      end
     end
 
     def self.attach_to(*)